Arsenic-based bacteria point to new life forms

We could be witnessing the first signs of a "shadow biosphere" – a parallel form of life on Earth with a different biochemistry to all others. Bacteria that grow without phosphorus, one of the six chemical elements thought to be essential for life, have been isolated from California's Mono Lake. Instead of phosphorus, the bacteria substitute the deadly poison arsenic.

"After one year, they are still alive and well," says Paul Davies of Arizona State University in Tempe. Not only that, the team showed that this ability was incorporated deep into the molecular building-blocks of the bacterium, strain GFAJ-1 of the salt-loving Halomonadaceae family, right down to the DNA.

Life's backbone

Until now, all known life has been built around the six major chemical elements carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur – known as CHNOPS – which make up proteins, lipids and DNA. In all normal life forms, phosphorus is a major part of the backbone of the genetic material.

"It's the first time such a chemical substitution has been shown for DNA," says Philippe Bertin of the University of Strasbourg, France, who was not part of the team. "Possibly, it's a relic of an ancestral metabolism that was supplanted during evolution because using phosphorus was more stable and less toxic."

Despite surviving on arsenic for a year, the bacteria would still "prefer" to grow using phosphorus: biomolecules react more efficiently in water and seem to be more stable when constructed with phosphorus than arsenic. They only substitute arsenic if there is no alternative.

Steven Benner, a chemist from the Foundation for Applied Molecular Evolution in Gainesville, Florida, who works on alternative forms of DNA, is sceptical that the bacteria really do contain arsenic. "I doubt these results," he says, since in order to measure the modified DNA it has to be put into a water-containing gel, which would rapidly dissolve any arsenate molecules. Any hypothesis that arsenate might replace phosphate in biomolecules must take this into account, he says.

Shadow biosphere

Davies says that future work will address the stability-in-water issue, but argues that the discovery underlines the need to look further for the first true representatives of alternate life forms in Earth's shadow biosphere.

Where to search? Extreme and isolated ecological niches such as dry deserts or cold plateaus, boreholes in the mantle or deeper, contaminated lakes or deep-sea hydrothermal vents would be a good target. "It could also be that this 'weird life' is all around us, intermingled with carbon-based life. If so, it's going to be hard to detect, as we would have to find a way to first filter everything out," says Davies.

Arsenic-bacteria have implications for possible extra-terrestrial life, too. "If life started in more than one manner on our planet, it would be very peculiar to believe that other places in the universe are not teeming with it," says Davies. He says we should think carefully about which chemical elements to follow to find it.

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